This invention relates generally to electric motors and, more particularly, to methods and apparatus for reducing vibration in a rotor assembly for electric motors.
Electric motors are used in countless varieties and applications worldwide. Typically, the rotational force and torque generated within the motor is delivered by a rotor shaft. The torque generated is the product of current applied to the motor and an electromagnetic field maintained in a stator. When a rotor generated magnetic field enters a stator generated magnetic field the rotor tends to speed up, and when the rotor magnetic field leaves the stator magnetic field the rotor tends to slow down. The torque produced is therefore non-uniform, and known to those in the art as torque ripple or cogging. Torque ripple produces objectionable noise and vibration at the motor shaft in some applications.
One example of such an application occurs when a motor drives a fan. Imbalances in the fan, combined with torque ripple, produce vibrations which are transmitted to the motor and fan mounting. These vibrations produce undesirable noise. Continued exposure over time to such vibrations loosens motor and fan assemblies, and ultimately failure of the motor. Damping systems are typically employed to minimize the effects of the vibrational energy induced into the motor and fan system.
A motor that includes a laminated rotor mounting assembly facilitates reducing or eliminating torque ripple and vibrations produced in a rotor core is described. The motor includes a stator assembly and a rotor assembly within a housing. The rotor assembly includes a rotor shaft, a plurality of magnetic elements and a rotor mounting assembly therebetween. The rotor mounting assembly includes a pair of resilient rings and a laminated spacer. Each resilient ring includes an inner metal insert and a resilient insert. In the exemplary embodiment the resilient ring includes a laminated outer annular ring which attaches to the laminated spacer. In another embodiment the laminated spacer circumferentially encloses the resilient ring. The inner metal insert attaches to the rotor shaft and the magnetic elements attach to the outer cylindrical surface of the spacer.
During operation, the rotor assembly rotates to align with a magnetic field generated within the stator assembly. The non-uniform magnetic fields generate torque ripple in the rotor core. The resilient inserts of the rotor mounting assembly damp vibrations and noise that may be generated as a result of such torque ripple. Reductions in torque ripple reduce vibrations and noise of the motor. As a result, more complex and expensive damping systems may be eliminated. The laminated outer annular ring and laminated spacer provide a reliable and cost-effective interface between the resilient inserts and the plurality of magnetic elements.